The present invention relates to a sight glass assembly suitable for use with a large vessel containing fluid under pressure. The fluid may be corrosive and/or at a high temperature.
Many prior art sight glasses consist of a single piece of glass of suitable composition supported in a mounting assembly including flange rings which are clamped against the outer and inner surfaces of the sight glass through manual tightening of flange bolts to seal the mounting assembly against leakage of pressure or vacuum. To insure a perfect seal the flange bolts are tightened which in turn cause uneven stresses in the glass lens. Another type of sight glass commonly in use with vessels containing fluids under pressure and at a high temperature consist of a single piece of glass of suitable composition mounted in the wall of said vessel in a suitable holder. Even though the composition and geometry of the glass selected resists the normal stresses caused by operating pressure and temperature it does not offer protection against the abnormal stresses produced by corrosion, impact, thermal shock, and lens scratches.
Another type of sight glass commonly available uses tempered glass lens. A tempered glass consists of an inner tensile layer and a thin outer compressive layer. This increases the strength of the glass and correspondingly increases its ability to withstand internal pressure, provided the thin compressive layer is not damaged by chemical corrosion or by scratches. If this occurs, the glass is actually weaker than an untempered piece since the undamaged glass is already in tension.